I've always just felt like people are just echoing Greg when they say Matau should've turned. Why do you think that? I've always thought Vakama made more sense than Matau, and Vakama was a pretty dumb choice. Out of all the Toa Metru Onewa was the best choice in my eyes. He was always fighting with the others, and even when he became better friends with the others his relationship with Vakama only got worse. In Maze of Shadows the two were constantly arguing, and Onewa tried to physically fight him a couple times. Vakama was so determined to save the Matoran he chose to keep pushing through the tunnels when Nokama was dying, and that pissed Onewa off like crazy. That was one of the times he tried to go after Vakama. In my eyes at least it sounds like Onewa just having enough and leaving makes sense, especially when they were mutated and started getting all emotional.
The whole arc was just poorly done. It should've just been Toa v Toa, not Toa v crazy Toa who wants to become the leader of an army of murderous spiders. Onewa just ditching the team to do things on his own, the other Toa having to deal with Vakama getting more violent and caring more about the Matoran than his own team...it could've taken a really dark path with all the development they put Vakama through in 04.